Coal is a primary source of energy today and is commonly used as fuel to produce electricity. A byproduct of producing electricity in a coal combustion process is nitrous oxide (NOx), which is emitted with a flue gas from coal burning electrical generating plants. This nitrous oxide is considered a pollutant to the atmosphere. Catalytic reactors are used to address this type of pollution by reducing the nitrous oxide concentration in the flue gas. Ash is another byproduct of burning coal and typically comprises silicon dioxide, calcium oxide, carbon and many other constituents depending on the makeup of the coal being burned. The combustion ash particles are usually small (up to 300 micro meters in diameter) and usually suspended in the flue gas. However, the combustion ash particles can form large particle ash (LPA), which may have a diameter exceeding 1 centimeter. LPA formation can be traced to combustion conditions in the boiler and clay like fly ash deposits on superheater tubes and backpass. The catalytic reactors are equipped with a plate or honeycomb-type catalyst and may have a pitch or opening ranging up to 8 millimeters. LPA particles are larger than the catalyst opening and therefore clogs up the catalytic reactor. As such, methods for separating ash from flue gas have been developed.
For example, U.S. Pat. No. 7,531,143, entitled “Arrangement for separating coarse ash out of a flue gas stream,” discloses screens with pleated arrangements for separating ash particles from flue gas. In practice, these screens experience blockage in certain areas of the pleated arrangement, which creates high velocity zones that cause damage to the screens. As a result, these pleated screens have to be replaced frequently or perform inefficiently and increase pressure drop in the system. It should also be noted that channels through which flue gas flows are large and, at least for this reason, screens used in these channels to separate ash from flue gas are also large and can be relatively costly.
U.S. Pat. No. 7,556,674, entitled “Method and device for the separation of dust particles,” discloses a system involving a baffle arrangement for deflecting ash particles from the flue gas towards hoppers, which collect the ash particles. This system requires a long duct to settle out the ash particles. The length of the duct makes this system relatively expensive.
The pleated screen design and baffle arrangement utilizing gravimetric forces do not economically remove large particle ash. Another problem in the art is that the ash particles in the flue gas erode structural duct members and separation screens. High flue gas velocities combined with hard ash particles will lead to significant metal wastage of this equipment. In summary, existing systems and methods for screening ash particles from flue gas are associated with high operating costs and high capital expenditure.